1. Field of the Invention
The present invention relates to a portable radiation image taking apparatus which is constructed such that a grid and a photo timer are attachable to the outside of a case.
2. Related Background Art
Up to now, in general, an apparatus that emits a radiation to an object and detects an intensity distribution of the radiation transmitted through the object to obtain a radiation image of the object has been widely used in an industrial nondestructive testing and a field of medical diagnosis. As a general method for the above image taking, there is an X-ray film/screen method. This is a method of conducting image taking using a combination of a photosensitive film and a phosphor sensitive to an X-ray. A sheet-shaped rare earth phosphor that emits light when it is irradiated with the X-ray is held in contact with both surfaces of the photosensitive film. The X-ray transmitted through the object is converted into visible light by the phosphor and the visible light is captured in the photosensitive film. A latent image formed on the photosensitive film can be visualized by development using chemical treatment.
On the other hand, along with a recent progress in digital techniques, a method of converting a radiation image into an electrical signal, image-processing the electrical signal, and then reproducing the image-processed electrical signal as a visible image on a CRT or the like to obtain a high quality radiation image is required. With respect to such a method of converting the radiation image into the electrical signal, there has been proposed a radiation image recording and reproducing system as described in JP 55-012429 A, JP 56-011395 A, or the like, in which a radiation transmission image is temporarily stored as a latent image in the phosphor and the phosphor is irradiated with excitation light such as laser light later to photoelectrically read out the latent image and output it as a visible image.
Also, along with a recent progress in semiconductor process techniques, an apparatus that similarly takes a radiation image using a semiconductor sensor has been developed. Because such a system has a much wider dynamic range than that in the conventional radiation photography system using a photosensitive film, there is an economic advantage that a radiation image which is not influenced by a variation in radiation exposure amount can be obtained. Simultaneously, as compared with the conventional photosensitive film method, there is an advantage that chemical treatment is unnecessary and an output image can be immediately obtained.
FIG. 7 shows a conventional example of such a radiation image taking apparatus. An image taking unit 103 of an X-ray image taking apparatus includes an X-ray detection sensor 104. An object 102 is irradiated with an X-ray generated by an X-ray generating apparatus 101. The X-ray transmitted through the object 102 is detected by photoelectric conversion elements which are arranged in two-dimensional grid in the X-ray detection sensor 104. An image signal outputted from the X-ray detection sensor 104 is processed into a digital image by an image processing unit 107 and an X-ray image of the object is displayed on a monitor 108.
A scattered X-ray removing grid (hereinafter referred to as a grid) 105 is provided in the inner portion of the image taking unit 103. The grid 105 is used for removing a scattered X-ray produced in the inner portion of the object (for example, a human body) by the X-ray irradiation to improve a contrast of the X-ray image. In the case where image taking is conducted, the grid 105 is located between an X-ray tube and a detector such as a photosensitive film.
FIG. 8 is a schematic sectional view of the grid. The X-ray is emitted from the A-direction on the left side in FIG. 8. In the grid 105, foils 201 made of a material having a large X-ray absorbency index and intermediate materials 202 having a small X-ray absorbency index are alternately laminated. In general, lead is used for the foils 201 having the large X-ray absorbency index. In addition, aluminum, paper, wood, a synthetic resin, a carbon fiber reinforced resin, or the like is used as the intermediate material 202 having the small X-ray absorbency index. The periphery of the laminate is covered with a cover made of aluminum, a carbon fiber reinforced resin, or the like, which is indicated by reference numeral 200.
In many cases, the grid 105 is a convergent grid. That is, a central foil which is represented by reference symbol 201a and located immediately under an X-ray source (the X-ray generating apparatus 101) is perpendicular to the cover 200. Foils represented by foils 201b are slanted at larger angles toward the X-ray source as their positions are closer to the end of the grid. In the case of the convergent grid, it is necessary to conduct image taking while adjusting a distance between the grid and the X-ray source and aligning the center of the grid and the center of the X-ray source with each other. On the other hand, there is a grid in which the foils are not slanted. This is called a linear grid.
Also, a photo timer for X-ray dose measurement (hereinafter referred to as a photo timer) 106 is provided in the inner portion of the image taking unit 103. The purpose of using the photo timer 106 is to measure a dose of an X-ray which actually reaches the detector (the X-ray detection sensor 104) in order to obtain a preferable image by adjusting it to a desirable X-ray dose. In addition, the purpose of using the photo timer 106 is to prevent a person to be examined from being exposed to a large dosage of X-rays. The photo timer 106 is used for terminating the generation of the X-ray in the X-ray generating apparatus 101 when the measured X-ray dose reaches a predetermined value. Therefore, the photo timer 106 is located between the grid 105 and the detector (X-ray detection sensor 104).
Up to now, such a kind of image pickup apparatus has been placed in a radiation room and used therein. However, in recent years, in order to enable image taking at a high speed and on a wide range section, a potable type image taking apparatus (which is also called an electronic cassette) is desired.
It is required that such an electronic cassette is thin and light in weight and has a high mechanical strength. In the image taking using the cassette, there is the case where the person to be examined mounts the cassette. In addition, there is a fear that an impact such as a drop or a collision is applied to the cassette due to its portability. Therefore, it is necessary to greatly improve a mechanical strength as compared with a conventional stationary image taking unit.
A schematic structure of the electronic cassette will be described with reference to a side cross section shown in FIG. 9. With respect to the electronic cassette used for the above-mentioned X-ray image taking, an X-ray detection panel 111 is two-dimensionally fixed onto a base 112 made of a metal in order to improve a strength with respect to a static pressure and bending. Further, an outer covering unit 114 is constructed so as to store the X-ray detection panel 111, the base 112, a circuit board 113 that processes electrical signals, and the like. In order to obtain a mechanical strength, a metal is used also for the outer covering unit 114. Note that, because a reduction in weight is also required, a light metal such as aluminum or magnesium is employed. In addition, a cover 115 made of a material such as a CFRP having preferable X-ray permeability is provided on an X-ray incident surface side.
A scattered radiation amount depends on the structure of the object. For example, in the case of a human body, the scattered radiation amount is large in the chest and the abdomen and is relatively small in the arms and legs. Here, because the grid reduces an X-ray transmitting therethrough, the image taking without using the grid can be conducted at a dose smaller than in the case where the image taking is conducted using the grid. Therefore, in many cases, the grid is used for conducting the image taking on a section in which the scattered radiation amount is large because an image quality is given a high priority, and the image taking without the grid is conducted on a section in which the scattered radiation amount is relatively small in order to reduce a dose. In addition, an optimum grid specification is changed according to a section.
It is considered that the cassette is used for various sections. Accordingly, it is desirable that the grid is constructed so as to be easily detachably attachable according to a section on which the image taking is conducted. In the case of the conventional stationary type, a detachably attachable mechanism for the grid can be constructed in the image taking unit without causing a problem. However, in the case of the electronic cassette, it is disadvantageous that an opening portion for grid attachment and detachment is provided in a portion of the outer covering unit in view of a strength.
On the other hand, in the case of the chest and the abdomen, a dose of the X-ray which reaches the X-ray detection panel 111 is generally changed depending on the body shape (fat content) of the person to be examined. Therefore, the photo timer is used in many cases. However, in the case of the image taking of the arms and legs and the like or the cassette image taking in doctor's rounds or the like, the frequency in use of the photo timer is low. Thus, it is desirable that the photo timer for the electronic cassette is detachably attachable. In addition, if the photo timer is attached, the thickness of the entire cassette increases by the thickness of the photo timer. Thus, in the case where the cassette is placed below the person to be examined, it is not desirable to attach the photo timer to the cassette.
In the conventional techniques as described above, it is desirable to provide a radiation image taking apparatus which is constructed such that a grid and a photo timer are selectively detachably attachable to an image taking unit and includes the image taking unit which is thin and light in weight, and has a high mechanical strength with respect to a static pressure, bending, an impact, and the like.